I've written an experimental Python Online Judge at http://pyonlinejudge.appspot.com/ . For the time being I'm seeing whether there's any interest in the project, and hoping people will test it (and see whether the concept works), so all logged in users have admin rights to create/edit tasks. Please play a bit with it if you're interested. Just don't delete existing data please.
If you have any suggestions or comments, or if you have tasks that you wish to add there (permanently, right now it's just for testing). please contact me.
As to why I wrote it... well it's implemented in Google's App Engine. I haven't been playing with online judges for quite a few years but it dawned on me that App Engine provides a nice environment to run arbitrary python code since that's what they do essentially. Just couldn't resist